1. Field of the Invention
The present invention relates to polymeric materials, preferably polypropylene based polymeric materials, having improved impact resistance and, more particularly, relates to visbroken reactor blends of free radical crosslinkable thermoplastic olefin polymers, such as thermoplastic polyethylene materials, and free radical decomposable olefin polymers, such as polypropylene materials, preferably high molecular weight polypropylene materials. The visbroken reactor blends of the present invention manifest high resistance to impact at low temperatures and have excellent stiffness and flow characteristics. Most importantly, the subject visbroken reactor blends comprise a continuous olefin polymeric phase and a dispersed thermoplastic olefin polymeric phase wherein improved adhesion characteristics are achieved between the dispersed and continuous phases, due in part to the presence of grafts. The grafts comprise a portion of free radical decomposable olefin polymer fragmented, or visbroken, material grafted to a portion of the free radical crosslinkable thermoplastic material. These grafts serve as compatibilizers or surface active agents between the phases. In a preferred embodiment, the free radical decomposable olefin polymer is polypropylene and the free radical crosslinkable thermoplastic olefin polymer is polyethylene. Preferably, the polypropylene material utilized is a high molecular weight material so that more polypropylene fragments are available for grafting and a larger amount of surface active grafts are thus provided to improve adhesion between the dispersed and continuous phases. The visbroken reactor blends of the present invention are particularly characterized by the fact that they do not include rubber materials yet manifest significantly improved low temperature impact resistance while maintaining excellent stiffness characteristics.
2. Prior Art
The present invention resides in the discovery that improved impact-modified polypropylenes are obtained by visbreaking a reactor blend of polypropylene and thermoplastic polyethylene. Polypropylene is known to be deficient in resistance to impact at low temperatures, i.e., 5.degree. C. and below. Polypropylene materials which have been modified to improve resistance to impact are generally referred to as impact-modified polypropylenes. Heretofore, impact resistance of polypropylene has been improved by blending impact resistant materials such as elastomeric ethylene-propylene copolymers therewith or by reacting therewith other polymers or copolymers to produce reactor blends typically referred to as impact copolymers. There is, however, a tradeoff with other properties, e.g., stiffness and flow characteristics, when such polypropylene materials are impact-modified. For example, U.S. Pat. No. 4,459,385 discloses that impact-modified polypropylenes obtained by sequential polymerization of propylene, and ethylene and propylene, to obtain an impact copolymer of polypropylene homopolymer and elastomeric ethylene-propylene copolymer, while having good impact resistance at low temperatures, have decreased stiffness. This reference teaches that as a general rule, impact resistance increases and stiffness decreases with increasing amounts of elastomer in the total composition. Such patent also discloses that the decrease in stiffness can be compensated for by blending with the impact-modified material a linear low density polyethylene (LLDPE) having a melt flow index not exceeding 5 dg/min. U.S. Pat. No. 4,535,125 discloses that impact-modified polypropylenes blended with a LLDPE have poor flow performance and that such flow performance is improved through visbreaking. That is, the melt flow rate of the blend of impact copolymer and LLDPE can be increased by extruding the blend in the presence of certain peroxide materials. Thus, impact-modified polypropylenes are blended with a LLDPE having a melt flow index of from 5 to 50 dg/min and the blends are visbroken to obtain impact-resistant polypropylene materials which have good stiffness characteristics as well as good flow characteristics.
Other methods of preparing impact-modified polypropylenes are also known. For example, U.S. Pat. No. 4,375,531 discloses blends of a first component selected from a group of medium impact polymeric materials with a second component selected from a group of high impact polymeric materials which blends are visbroken to improve the flow characteristics thereof.
It can be seen from the prior art that impact resistant polypropylenes having good flow and stiffness characteristics are prepared starting with polypropylene materials which have already been impact-modified, i.e., contain elastomers and the like, and blending therewith another material such as LLDPE to improve the stiffness characteristics of the impact-modified polypropylene followed by visbreaking the blend to improve the flow characteristics of the blend. However, such products manifest an undesirable balance between impact resistance and stiffness. Furthermore, products produced from such materials exhibit "blushing" or "stress whitening" upon impact which gives an appearance of a defective product.
It has now been discovered that, contrary to the teachings of the prior art, high-impact resistant polypropylene materials which do not contain rubber materials, such as ethylene-propylene rubber, and which do not contain stiffness modifiers, such as LLDPE, yet have excellent flow and stiffness characteristics as well as superior antiblush characteristics, are obtained by visbreaking reactor blends of thermoplastic polyethylene and polypropylene materials preferably prepared in the absence, or essential absence, of chain transfer agents such as hydrogen. That is, by restricting the flow of hydrogen, or other chain transfer agents typically utilized to control melt flow rate during sequential polymerizations, and subsequently visbreaking the reactor blended copolymers or sequentially polymerized copolymers, impact resistant polypropylene materials having superior impact resistance, stiffness and antiblush characteristics can be produced without adding an impact modifying component such as EP rubber.
Accordingly, the subject invention provides high impact resistant visbroken reactor blends suitable for use in various applications, such as in molded products, wherein the products produced are significantly superior than those of the prior art. In addition, the improved impact-modified polymers of this invention are prepared in a more convenient manner and at a substantial cost savings as compared to the prior art materials.